Name

    ARB_draw_elements_base_vertex

Name Strings

    GL_ARB_draw_elements_base_vertex

Contributors

    Daniel Koch, TransGaming
    James Helferty, TransGaming
    Jeff Bolz, NVIDIA
    Bruce Merry, ARM
    Ian Romanick, Intel
    Jon Leech, Khronos

Contact

    Daniel Koch, TransGaming (daniel 'at' transgaming.com)

Notice

    Copyright (c) 2009-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete. Approved by the ARB on July 3, 2009.

Version

    Last Modified Date:  August 2, 2009
    Version:             3

Number

    ARB Extension #62

Dependencies

    This extension is written against the OpenGL 3.1 Specification but
    can apply to prior specifications.

    This extension interacts with ARB_draw_instanced.

    This extension interacts with EXT_draw_instanced.

    This extension interacts with ARB_instanced_arrays.

    This extension interacts with ARB_compatibility.

Overview

    This extension provides a method to specify a "base vertex offset"
    value which is effectively added to every vertex index that is
    transferred through DrawElements.

    This mechanism can be used to decouple a set of indices from the
    actual vertex array that it is referencing. This is useful if an
    application stores multiple indexed models in a single vertex array.
    The same index array can be used to draw the model no matter where
    it ends up in a larger vertex array simply by changing the base
    vertex value. Without this functionality, it would be necessary to
    rebind all the vertex attributes every time geometry is switched and
    this can have larger performance penalty.

    For example consider the (very contrived and simple) example of
    drawing two triangles to form a quad. In the typical example you
    have the following setup:

          vertices                indices
         ----------                -----
      0 | (-1,  1) |            0 |  0  |
      1 | (-1, -1) |            1 |  1  |
      2 | ( 1, -1) |            2 |  2  |
      3 | ( 1,  1) |            3 |  3  |
         ----------             4 |  0  |
                                5 |  2  |
                                   -----
    which is normally rendered with the call

       DrawElements(TRIANGLES, 6, UNSIGNED_BYTE, &indices).

    Now consider the case where the vertices you want to draw are not at
    the start of a vertex array but are instead located at offset 100
    into a larger array:

           vertices2             indices2
           ----------             -----
              ....             0 | 100 |
      100 | (-1,  1) |         1 | 101 |
      101 | (-1, -1) |         2 | 102 |
      102 | ( 1, -1) |         3 | 103 |
      103 | ( 1,  1) |         4 | 100 |
              ....             5 | 102 |
           ----------             -----

    The typical choices for rendering this are to rebind your vertex
    attributes with an additional offset of 100*stride, or to create an
    new array of indices (as indices2 in the example). However both
    rebinding vertex attributes and rebuilding index arrays can be quite
    costly activities.

    With the new drawing commands introduced by this extension you can
    instead draw using vertices2 and the new draw call:

       DrawElementsBaseVertex(TRIANGLES, 6, UNSIGNED_BYTE, &indices, 100)

New Procedures and Functions

    void DrawElementsBaseVertex(enum mode, sizei count, enum type,
         void *indices, int basevertex);

    void DrawRangeElementsBaseVertex(enum mode, uint start, uint end,
         sizei count, enum type, void *indices, int basevertex);

    void DrawElementsInstancedBaseVertex(enum mode, sizei count,
         enum type, const void *indices, sizei primcount, int basevertex);

    void MultiDrawElementsBaseVertex(enum mode, sizei *count, enum type,
         void **indices, sizei primcount, int *basevertex)

New Tokens

    None

Additions to Chapter 2 of the OpenGL 3.1 Specification (OpenGL Operation)

    Add the following to the end of Section 2.8.1 "Transferring Array Elements"

    "When one of the *BaseVertex drawing commands specified in section
    2.8.2 is used, the primitive restart comparison occurs before the
    <basevertex> offset is added to the array index."

    Add the following to Section 2.8.2 "Drawing Commands"

    "The commands
      void DrawElementsBaseVertex(enum mode, sizei count, enum type,
           void *indices, int basevertex);

      void DrawRangeElementsBaseVertex(enum mode, uint start, uint end,
           sizei count, enum type, void *indices, int basevertex);

      void DrawElementsInstancedBaseVertex(enum mode, sizei count,
           enum type, const void *indices, sizei primcount, int basevertex);

    are equivalent to the commands with the same base name (without the
    "BaseVertex" suffix) except that the <i>th element transferred by
    the corresponding draw call will be taken from element
       <indices>[<i>] + <basevertex>
    of each enabled array. If the resulting value is larger than the
    maximum value representable by <type> it should behave as if the
    calculation were upconverted to 32-bit unsigned integers (with
    wrapping on overflow conditions). The operation is undefined if the
    sum would be negative and should be handled as described in Section
    2.9.2. For DrawRangeElementsBaseVertex, the index values must lie
    between <start> and <end> inclusive, prior to adding the
    <basevertex> offset. Index values lying outside the range
    [<start>,<end>] are treated in the same way as DrawRangeElements.

    The command

      void MultiDrawElementsBaseVertex(enum mode, sizei *count,
           enum type, void **indices, sizei primcount, int *basevertex);

    behaves identically to DrawElementsBaseVertex except that
    <primcount> separate lists of elements are specified instead. It has
    the same effect as:

      for (i = 0; i < primcount; i++) {
        if (count[i] > 0)
          DrawElementsBaseVertex(mode, count[i], type, indices[i],
                                 basevertex[i]);
      }"


    In Section 2.9.5 "Array Indices in Buffer Offer Objects" add
    references to the new drawing commands.

    In the third paragraph, replace the second sentence (which begins
    with "MultiDrawElements also sources...") with the following
    sentences:

    "DrawElementsBaseVertex, DrawRangeElementsBaseVertex, and
    DrawElementsInstancedBaseVertex also source their vertices from that
    buffer object, adding the <basevertex> offset to the appropriate
    vertex index as a final step before indexing into the vertex buffer;
    this does not affect the calculation of the base pointer for the
    index array. Finally, MultiDrawElements and
    MultiDrawElementsBaseVertex also source their indices from that
    buffer object, using its <indices> parameter as a pointer to an
    array of pointers that represet offsets into the buffer object."

Additions to Chapter 3 of the OpenGL 3.1 Specification (Rasterization)

    None

Additions to Chapter 4 of the OpenGL 3.1 Specification (Per-Fragment
Operations and the Frame Buffer)

    None

Additions to Chapter 5 of the OpenGL 3.1 Specification (Special
Functions)

    None

Additions to Chapter 6 of the OpenGL 3.1 Specification (State and
State Requests)

    None

Additions to the AGL/GLX/WGL Specifications

    None

Dependencies on OpenGL 3.1

    If OpenGL 3.1 is not supported, ignore all references to
    DrawElementsInstanced and DrawElementsInstancedBaseVertex

Dependencies on the ARB_draw_instanced extension

    If ARB_draw_instanced is supported, the functionality provided by
    DrawElementsInstancedBaseVertex can also be described in terms of
    DrawElementsInstancedARB instead of DrawElementsInstanced.

Dependencies on the EXT_draw_instanced extension

    If EXT_draw_instanced is supported, the functionality provided by
    DrawElementsInstancedBaseVertex can also be described in terms of
    DrawElementsInstancedEXT instead of DrawElementsInstanced.

Dependencies on the ARB_instanced_arrays extension

    If ARB_instanced_arrays is supported, the functionality provided by
    DrawElementsInstancedBaseVertex can also be described in
    terms of DrawElementsInstancedARB instead of DrawElementsInstanced.

Dependencies on the ARB_compatibility extension

    When the ARB_compatibility extension is supported, the base vertex
    functionality applies to both buffer objects and client-side vertex
    arrays. Additionally there may be some textual differences in the
    specification because the behaviour of DrawElements is defined in
    terms of ArrayElement, but the functionality remains the same. Note
    in particular the interaction with the primitive restart index as
    identified in Issue 5.

    When ARB_compatibility is supported edit the first bullet point of
    the "Shader Inputs" subsection of Section 2.14.7 "Shader Execution"
    and replace the language enumerating the drawing commands which
    specify a complete primitive for the purposes of defining
    gl_VertexID to be more general purpose:
        "(a vertex array drawing command other than ArrayElement)."

Errors

    The *BaseVertex commands have identical error conditions to the
    non-*BaseVertex functions, and all values of <basevertex> are legal
    (with the exception of ones which cause accesses outside of vertex
    arrays or bound buffers as described in Section 2.9.2).

New State

    None

New Implementation Dependent State

    None

Issues

    1. What should this extension be called?

       RESOLVED: Using ARB_draw_elements_base_vertex.

       DISCUSSION: Using the base "draw_elements" since this extension
       adds a new variant to the DrawElements family of commands which
       takes an additional parameter. Since the new suffix on the
       drawing commands is "BaseVertex" it makes sense to call this
       "draw_elements_base_vertex" (and it is more aesthetically
       pleasing then "draw_elements_basevertex".

       Initial versions of this extension were called
       "draw_elements_offset" (see Issue 9).

       Other alternatives considered: index_offset or element_offset.
       These variants might have been more suitable if we had used a
       different mechanism for specifying the base vertex offset (see
       Issue 2).

    2. Should we have a per-draw call parameter or should this be
       specified via some other mechanism (ELEMENT_ARRAY bind parameter,
       global state, etc).

       RESOLVED. Using per-draw call.

       DISCUSSION: If per-draw call we need entry points to specify the
       equivalent of DrawElements, DrawRangeElements,
       DrawElementsInstanced and possibly MultiDrawElements, but with an
       additional parameter.

       Per binding point, such as
       glBindBufferBaseVertex(ELEMENT_ARRAY_BUFFER, id, 1234)? If
       per-ELEMENT_ARRAY binding point, the application will need to
       rebind the index buffer every time they wish to adjust the
       basevertex, which partially defeats the purpose of this
       extension. As well this would make it more difficult to support
       client arrays in an implementation which supports
       ARB_compatibility.

       If this is a global state, should it be server or client state?
       Is it per-gc? We could have a separate API call, e.g.
       glIndexOffset(1234), that would be per-VAO. This method doesn't
       require rebinding buffers and would work fine for client vertex
       arrays. In many ways though having a global state doesn't make
       much sense, since it is may only relevant for a single draw call,
       and encapsulating this in the VAO-state may mean that additional
       VAO must be created.

    3. This functionality seems vaguely familiar. Do any other APIs have
       this functionality?

       YES. This is equivalent to the BaseVertexIndex (d3d9) or
       BaseVertexLocation (d3d10) indexed drawing parameters in
       Direct3D.

    4. Should there be a MultiDrawElementsBaseVertex? If so, should it
       take a single basevertex or an array of them?

       RESOLVED: YES. Let's add it for completeness. It seems to make
       the most sense to pass in an array of basevertex parameters for
       this command, as this provides the most flexibility, and it works
       well to define it in terms of DrawElementsBaseVertex.

    5. What are the interactions with primitive restart?

       RESOLVED. The primitive restart comparison occurs before adding
       the basevertex.

       DISCUSSION:
       DX10 and existing hardware do the primitive restart comparison
       before adding the basevertex. It's really the only sane thing to
       do, otherwise the app would have to change the restartindex to
       depend on the basevertex.

       This is counterintuitively not the result you would get if
       DrawElements were still defined in terms of ArrayElement and you
       defined these in the natural way
       (ArrayElement(indices[i]+basevertex)), but this is likely what
       developers want and also what the hardware does.

    6. What happens if indices[i]+basevertex is larger than the max
       value representable by <type>.

       RESOLVED. Behave as if everything is upconverted to 32-bit
       unsigned integers. If the addition over/underflows 32-bits, it
       wraps. This is the behaviour D3D10 uses and likely how it is
       implemented in hardware.

    7. What happens if the sum is negative (ie indices[i]+basevertex < 0)?

       RESOLVED: Undefined. This should be handled the same way as a
       buffer that accesses out of bounds data. This is defined in
       Section 2.9.2 "Effects of Accessing Outside of Buffer Bounds". If
       detected this results in a GL error, otherwise it has undefined
       results and may result in GL interruption or termination. This is
       also undefined under Direct3D.

    8. For DrawRangeElementsBaseVertex, is the intent that indices[i]
       are all in [start,end] and not indices[i]+basevertex?

       RESOLVED: YES, indices[i] must be in the range [start,end].

       DISCUSSION: There doesn't appear to be a hardware or driver
       reason to prefer one interpretation over the other. D3D9 treats
       the MinIndex parameter to the DrawIndexedPrimitive call as
       relative to the index buffer, so in the interests of
       compatibility we will make the same choice.

    9. The word "offset" sounds like it should be measured in bytes. Is
       there a better term we could use for this?

       RESOLVED: Use "BaseVertex" and call the parameter <basevertex>.

       BACKGROUND:
       Initial drafts of this extension used the suffix "Offset" on the
       DrawElement calls, and the new parameter was named
       "vertexoffset".

       DISCUSSION: What other term could we use?

       Possibly something with "base" in it, partly because DX has that
       in the name and partly because it reminds one of ListBase and
       texture base level. However it does conflict with the meaning of
       "base" in BindBufferBase...

   10. Clarification is needed when using an ELEMENT_ARRAY_BUFFER, since
       <indices> is actually an offset into a VBO rather than an array
       itself.

       DISCUSSION: When ELEMENT_ARRAY_BUFFERS are used to provide the
       index data (as they must be in GL 3.1 without the
       ARB_compatibility extension) the <indices> parameters to the
       DrawElements calls are treated as offsets into the buffer objects
       as described in Sections 2.9.4 and 2.9.5.

       Logically this is as if a <realindices> array were computed as
       follows: <type> realindices = (<type>)((byte)bufferptr +
       ((byte)indices - (byte)NULL))

       Then base vertex offset is then computed as "realindices[i] +
       basevertex"

   11. Why do the new function entry points in this extension not have
       the "ARB" suffixes like other ARB extensions?

       RESOLVED: This extension is a strict subset of the functionality
       already in OpenGL 3.2. This extension exists only to support that
       functionality on older versions of OpenGL and on hardware which
       is not OpenGL 3.x capable. Since there are no possible behavior
       changes between the ARB extension and core features, source code
       compatibility is improved by not using suffixes on the extension.

Revision History

    Rev.    Date    Author     Changes
    ----  -------- ---------  ----------------------------------------
     0.1   4/29/09  dgkoch     initial skeleton
     0.2   4/30/09  dgkoch     first cut at per-draw call spec
     0.3   4/30/09  dgkoch     add issues 4-10, fix some typos and error in the example
                               based on comments from JB, BM
     0.4   5/06/09  dgkoch     resolved issue 2, sticking with per-draw call
                               resolved issue 4, add MultiDrawElementsBaseVertex w/ array
                               resolved issue 5, primitive restart compare happens first
                               resolved issue 6, behave as if 32-bit, including wrapping
                               resolved issue 7, undefined behaviour
                               resolved issue 8, indices[i] lies in the range [start,end]
                               sugggested resolution for issue 9: "BaseVertex"
                               clarified issue 10
                               misc formatting and typo fixes
     0.5   5/07/09  dgkoch     flipped y-coord in example
                               fixed spacing and typos
                               added more disscussion to Issue 2
                               removed clause from Issue 7 which contradicted Issue 6
     0.6   5/15/09  dgkoch     add resolution to issue 9, and rename appropriately
                               resolve issue 1
                               add interactions/dependencies
                               minor updates to the overview
                               update textual edits to reflect issues 5-8, 10
                               add issue 11
     0.7   5/18/09  dgkoch     minor grammer, spelling and typographical errors
                               Add interaction with ARB_compatibility
     1     6/26/09  dgkoch     resync language with GL3.2 spec
                               Add more interactions with ARB_compatibility
     2     7/21/09  dgkoch     resync language with 20090630 3.2 spec
     3     8/02/09  Jon Leech  Reformat to 80 columns and assign ARB
                               extension number.